My air compressor motor is a 3hp/230v single phase. The plug is a 3-prong marked 250v. I've checked the L1 lead (black) at the motor proper and show continuity (ohmmeter) to the L2 lead (white). I wired the receptacle using a 20 amp breaker; running the white and black leads of 12/2 wire from the breaker to the L1 and L2 connections on the outlet with the ground (copper) going to the ground lug on the outlet. The motor does nothing wired this way. Move the white lead at the breaker to the ground bar in the panel and the motor will run but can't handle any load whatsoever which I understand is the norm for a 230v motor being run on 120v. What am I doing wrong?
You most likely have the black and white phase conductors on the same busbar in the panel.
I'm going to assume that this is a residential installation with 120V / 240V single-phase power.
Take a close look at your breaker panel. You will see the two main bus bars running the length of the panel where the breakers mount. You need to ensure that one side of the breaker connects to one bus bar and the other side of the breaker to the other bus bar.
Most breaker panels are manufactured so that so that adjacent breakers are always on opposite phases. However, some older panels were built so that two adjacent breaker slots are on one phase, with the next pair of breaker slots on the other phase. This pattern repeats over the entire length of the breaker panel.
This was done to accommodate large breakers (60 Amps or more).
Simply move your double breaker up or down one slot and you should be good to go.